How to grow onion

Know your daylength. Near the equator (low latitude) the days stay close to 12 hours all year — you need a short-day variety. Further from the equator, where summer days are long, an intermediate or long-day type may fit.

Buy a named short-day (tropical) variety from a trusted dealer — a labelled packet with the variety name on it, not loose seed scooped from a tin.

Check the seed is fresh — this season's packing date. Onion seed dies quickly; seed more than about a year old comes up thin, patchy and weak.

If you are unsure which type suits your area, plant the variety your successful neighbours grow, or ask the dealer or extension officer for a short-day type by name.

About 6–8 weeks before you want to transplant, make a seedbed in full sun on well-drained ground. Raise the bed about 10–15 cm above the surrounding soil so water drains away — wet beds rot seedlings.

Get your seed ready: about 3–5 kg of seed for every hectare of field you plan to plant out — much less for a small garden plot. A little onion seed raises far more seedlings than it looks, so you can sow sparingly.

Break the soil to a fine, crumbly tilth like flour and work in a little well-rotted compost. Onion seed is tiny and needs fine soil pressed around it.

Draw shallow lines across the bed about 10 cm apart (half a hand-span) with a stick.

Sow the seed thinly along the lines — space it out, don't pour it. Crowded seedlings grow thread-thin and catch damping-off.

Cover the seed lightly — about 1–1.5 cm, no deeper than the width of your little finger — firm it gently, and water with a fine sprinkle (a can with a rose, not a splash).

Lay light shade over the bed — a thin layer of dry grass or a shade net — to stop the soil baking. Seedlings show in 4–7 days; lift the shade a little once they are up.

Keep the bed moist but never soggy, and pull weeds by hand while they are small.

In the last week before transplant, harden the seedlings off: give a little less water and more sun so they toughen. Transplant when they are pencil-thick at the base and about 12–15 cm tall.

About 1–2 weeks before transplanting, pick a well-drained, fertile field in full sun — never a low spot that holds water, which rots the shallow roots.

If your soil is sour (acid), below pH 6, lime it well ahead of time (lime works slowly) so the bed sits around pH 6–7, where onion feeds best.

Dig and break the soil to a fine, firm, level tilth and work in well-rotted manure or compost. Onion's small roots need fine, settled soil, not big clods.

Work in all the phosphorus (P), the first part of the nitrogen (N) and the potassium (K) — your recommendation gives the amounts — and mix them into the topsoil where the shallow roots will reach. (The sulphur (S) goes on later, with the main nitrogen top-dress — not now.)

Make beds or ridges you can water and weed easily, and firm the surface so seedlings sit snugly.

Buy pencil-thick seedlings, about 12–15 cm tall, with firm, upright leaves — not thin, thread-like or floppy ones.

Confirm with the seller that it is a named short-day variety suited to your daylength — the wrong type grows healthy leaves all season and never forms a bulb, and you won't know until it's far too late.

Check the batch for silvery streaks or white flecks on the leaves (thrips damage) and reject any plants that look wilted, spotted or diseased.

Transplant within a day or two of buying — keep the seedlings shaded and moist until you get them into the field.

Transplant in the cool of late afternoon or on a dull day, so the seedlings don't wilt in the midday sun.

Water the nursery bed first, then lift the seedlings gently with as much root as you can. Keep them in the shade and plant them the same day.

You may trim the very tips of the long leaves and any over-long roots with a clean knife — this cuts water loss and helps them stand up. Keep only pencil-thick seedlings; leave the thread-thin ones behind.

Set each seedling shallow — just deep enough to stand up, about 2–3 cm (no deeper than one finger-joint). Do not bury the growing point (the pale spot where the leaves fan out) — plant deep and the bulb comes out small and misshapen.

Space them about 20 cm between rows (a hand-span) and 5–10 cm apart in the row (a few fingers) — close spacing gives many medium bulbs; wider spacing gives fewer, larger ones.

Firm the soil around the roots with your fingers so no air pocket is left, then water in straight away.

Keep the soil moist every day for the first week or two while they take hold — you will see the centre leaf stand up and green up once they have rooted.

Water lightly every 2–3 days for the first two weeks — little and often, not one big flood.

Keep the top few centimetres moist; push a finger in and water before it dries out.

Water in the cool of morning or evening, not the hot midday, so less is lost and the plants don't scorch.

Don't waterlog — onion roots rot in standing water. If water pools, drain it; the soil should be damp, not swampy.

Twice a week, walk the rows and look closely at 25–50 plants across the whole field, not just the edge by the path.

Thrips: part the leaves and look in the crease where leaf meets leaf (the axil) — thrips are tiny, pale-yellow to brown slivers that scurry when disturbed, leaving silvery streaks and white flecks on the leaves. Start control at about 1 thrips per leaf (sooner in hot, dry weather).

Damping-off: look for seedlings that wilt and topple with a pinched, watery stem at the soil line, often in wet patches. Pull out the collapsed ones, ease off the water, and improve drainage so the surface can dry.

Note any gaps where seedlings died, so you can gap-fill from spare nursery seedlings while they can still catch up.

If thrips reach the threshold → spray a recommended product, rotating chemical groups. If damping-off is spreading → stop over-watering and dry the bed out.

Twice a week, check 25–50 plants spread across the field.

On each, part the leaves and count the thrips in the leaf axils (the creases) — tiny pale-yellow to brown slivers that dart away when disturbed.

Spray at about 1 thrips per leaf; act faster at 2 or more per leaf and in hot, dry weather when they multiply fast.

When you spray, rotate between different chemical groups each time — thrips build resistance quickly, and using the same product over and over stops it working.

Look too for purple, ringed blotches on the leaves (purple blotch) in warm, damp weather, and a purple-grey furry mould in cool, damp spells (downy mildew); treat at the first spots with a recommended fungicide.

If thrips are over threshold → spray now, rotating groups. If leaf disease appears → start a preventive fungicide and avoid wetting the leaves late in the day.

Start weeding early and often — go over the crop every couple of weeks, before the weeds get bigger than the onions.

Hoe shallowly or hand-pull close to the plants — onion roots sit just under the surface, and deep hoeing cuts them.

Pull weeds out by the root where you can; chopping only the tops lets many spring straight back.

Take care not to knock soil over the growing point or bruise the leaves as you work.

Keep the field clean right through leaf growth — a weedy onion crop never catches up.

Apply the main nitrogen top-dress during leaf growth — roughly 3–4 weeks after transplanting, when the plants are growing fast.

Add the crop's sulphur (S) with this feed (for example a sulphur-bearing fertiliser) — this is the one place sulphur goes on, and it builds flavour and storage life, not just size.

Band it to the side of the rows, a few centimetres from the plants — not on top of them — and water it in so it reaches the shallow roots.

Keep the fertiliser off the wet leaves, where it sticks and scorches; brush any grains off the foliage.

Plan for one more, final nitrogen side-dress as bulbing begins — then stop (next stage). Late nitrogen gives thick necks and soft bulbs that won't store.

Water to keep the soil evenly moist — check with a finger and water when the top few centimetres start to dry.

Aim for roughly every 4–6 days, more often on sandy soil and in heat, less in cool, cloudy spells.

Water steadily — avoid the swing from bone-dry to flooded, which stresses the shallow roots and can split bulbs later.

Don't waterlog; the soil should be damp, not swampy, and low spots should drain.

Keep checking 25–50 plants twice a week, right through bulbing.

Count thrips in the leaf axils — spray at about 1 per leaf, faster at 2 or more and in hot, dry weather, rotating chemical groups each time.

Look for purple, ringed blotches on the leaves (purple blotch) and a purple-grey furry mould in cool, damp, leaf-wet weather (downy mildew); apply a recommended fungicide at the first spots and repeat as advised.

Watch the necks: they should stay slim. Thick, soft necks are an early warning of too much late nitrogen and poor storage to come.

If thrips or disease are over threshold → treat now — rotate insecticide groups, start the fungicide early, spray in the cool with the wind behind you, and respect the days-to-harvest wait.

Give the final nitrogen side-dress as bulbing just begins — around 6–8 weeks after transplanting, when you first feel the base starting to swell.

Band it to the side and water it in to reach the shallow roots; keep it off the leaves.

Then stop all nitrogen. No more N for the rest of the season — this is the rule that decides whether your bulbs store.

From here, potassium and steady water do the work of filling firm, well-skinned bulbs.

Keep the soil steadily, evenly moist all through bulbing — never let it dry out.

Water roughly every 3–5 days, more often in heat and on sandy soil; check with a finger and don't wait for the plants to wilt.

Keep the watering even — a swing from dry to soaked makes bulbs split, crack or grow double.

Water in the cool of the day so less is lost to the sun.

Ease off only near the very end of this stage as maturity approaches (the next stage tells you when to stop).

Stop watering about 15–25 days before you expect to harvest — as the tops begin to yellow and flop.

Let the soil dry down from now on; the drying is what firms the bulb and tightens the neck.

Watch the tops fall over and the outer skins begin to colour and go papery.

Time it with the weather — you want the ground drying, not soaking, going into harvest.

Walk the field and look at how many tops have fallen over at the neck — bent right down, not just leaning.

When about half to three-quarters (50–75%) of the tops have flopped over, the crop is mature and ready to lift.

Feel a few bulbs — they should be full and firm, with the outer skins starting to go dry and papery.

Don't knock the tops down by hand to rush it — let them fall on their own; forced-over plants haven't finished filling.

If most tops are still standing upright → wait a few more days and check again. If most are down and the skins are papery → lift on the next dry day.

Lift on a dry, sunny day, when about 50–75% of the tops are down and the soil is dry.

Loosen the soil first with a fork or hoe, then ease the bulbs out by hand — don't yank them by the tops, which bruises the neck.

Handle them gently, like eggs — don't drop them into the basket; every knock starts a rot.

Shake off loose soil and lay the bulbs out to begin curing — briefly on the surface, or straight into a dry, shaded, airy spot in hot regions to avoid sun-scald.

Leave the tops and roots on for now — you will trim them only after the bulbs have cured.

Spread the lifted bulbs in a single layer (or hang them in bunches) in a dry, shaded, airy place — out of rain and direct sun.

Let plenty of air move around them — a raised rack, mesh or slatted floor is far better than a solid pile.

Cure for about 2 weeks in hot, dry weather, up to 4 weeks in humid weather.

Check for readiness: the crop is cured when the necks are tight, dry and crinkly and the outer skins are papery and rustle.

If the necks are still soft, thick or green → keep curing longer and improve the airflow before you store. Storing before the necks are dry invites neck rot.

Once cured, trim the dry tops about 2–3 cm above the bulb (leave a small dry neck — don't cut into the bulb) and trim the roots off close.

Grade the bulbs: set aside any that are soft, thick-necked, bruised, cut, sprouting or split — eat or sell those first; they will not keep.

Keep only firm, well-skinned, tight-necked bulbs for storage.

Store them cool, dry and well-aired — in nets, slatted crates or thin layers, off the floor, where air can move; never sealed in a wet sack.

Check the store regularly and pull out any bulb that softens, smells or starts to rot before it spreads.